The problem of transmitting wide band television and video image information over narrow band telephone lines was recognized at an early date for example by Espenschied et al. in U.S. Pat. No. 1,835,031 issued in 1931. Espenschied et al. noted for example in column 2 that transmission of television pictures or movies would require a bandwidth of for example at least 1 MHz while the frequency bandwidth of conventional cables at the time apparently did not exceed 5 KHz. Espenshied et al. in 1929 therefore invented the coaxial cable described in U.S. Pat. No. 1,835,031 to provide a wide band channel for coaxial cable guided transmission of video signals for television. Such coaxial cable however is not available over conventional telephone lines which continue to be even more severely limited in bandwidth by loading to the 4 KHz range.
A search was conducted in the patent records of the USPTO in subclasses related to two way audio and visual telephone communication systems, video telephone systems, and television/telephone systems. These patents are found in the telephone Class 379, Subclasses 53 and 54. Most of the patents in the pertinent Subclasses of the search describe television/telephone or video telephone systems which incorporate two separate transmission channels, a narrow bandwidth audio channel and a wide bandwidth video channel. Such systems are exemplified for example by the Anderson et al. U.S. Pat. No. 3,612,767 of Bell Telephone Laboratories, Inc. for a system with audio and wide band communication paths; the Bjork et al. U.S. Pat. No. 3,922,491 of Telefonaktiebolaget of Sweden for a telecommunication system with a wide band channel by the side of a telephone connection; Fossen et al. U.S. Pat. No. 3,662,110 of Bell Telephone Labs for a combined wide band narrow band communication; and the many patents on teleconferencing such as the Fabris et al. U.S. Pat. No. 4,516,156 of Satellite Business Systems of McLean Virginia for a teleconferencing method and system using wide band communication links; the Colton et al. U.S. Pat. No. 4,529,839 of AT&T Bell Laboratories for a multi-location video conference system using satellite channels, etc. The Strebel U.S. Pat. No. 4,530,084 of the Heinrich Hertz Institute of Berlin describes a communications network for audio and visual data on a wide band optical fiber channel.
A second group of patents describe techniques for reducing bandwidth by data compression. The Eastman Kodak U.S. Pat. Nos. 4,774,574 and 4,774,562 of Daly et al. and Chen et al. describe the use of spatial transform coding for image compression. However this cannot achieve the several orders of magnitude reduction required to transmit on the telephone lines. The Eastman Kodak Company systems similar to the Bell Telephone Laboratory PicturePhone (TM) systems can only achieve still image communication. The current television standard of 525 scan lines per frame is reproduced on the CRT television screen, with 30 or 60 frames per second. The television video signal requires a bandwidth in the order of 4 or 4.5 megahertz on a carrier frequency and this bandwidth cannot be reduced for transmission on a conventional telephone line. Bell Telephone Laboratories has apparently achieved signal compression from 4.5 to 1.5 MHz which can pass in pulse code modulation (PCM) on the T carrier associated with elaborate equipment between some branch exchanges. However even the compressed signal cannot pass on the conventional 4 KHz narrow band transmission telephone lines.
A third group of patents is directed to the so-called "slow scan" or "slow scanning" method of transmission of image information over the narrow bandwidth telephone lines. The limitations of the slow scan method for transmission of high resolution television images over the narrow bandwidth telephone lines is described in columns 1 and 2 of the Reiffel et al. U.S. Pat. No. 4,654,484 issued Mar. 31, 1987 and assigned to Interand Corporation. For high resolution images only still images may be transmitted. As stated in column 1: "The time necessary for present slow scan systems using current compression schemes to transmit sufficient information for a video presentation of acceptable quality requires in the order of 90 to 120 seconds per display frame at transmission rates of 9,600 bits per second." The conventional "PicturePhone" (TM) video telephone is therefore able to transmit still images only. The Bell Laboratories "PicturePhone" (TM) is further described in columns 1 and 2 of the Wendlind et al. U.S. Pat. No. 3,980,830. Reference is made to a further description in the Bell Technical Journal, Volume 50, at pages 235-269 (1971). This system provides still image transmission of a person's face apparently with satisfactory picture resolution.
Multiplexing of slow scan television signals and voice signals over a telephone line is described in the Cavanaugh U.S. Pat. No. 4,099,202 assigned to Robot Research Inc. of San Diego, Calif. Apparently at lower resolution the slow scan rate still requires approximately 8 to 32 seconds per video frame to provide an image of acceptable quality. This patent discusses the problems of communicating voice signals at the same time as the slow scan television still image transmission. Further variations on the slow scan picture telephones are described in the Lemelson et al. U.S. Pat. Nos. 4,258,387 and 4,485,400; the Cooper-Hart U.S. Pat. No. 4,715,059 for a conversational video phone assigned to Luma Telecom, Inc. of Santa Clara, Calif.; and the Danell et al. U.S. Pat. No. 3,976,831 of Telefonaktiebolaget, Stockholm Sweden for a picture telephone with stationary pictures.
A more complete comparison of the Bell System PicturePhone (TM) video telephone with commercial television standards is described in the Bell Telephone Laboratories U.S. Pat. No. 3,806,644 of Browne et al. Simultaneous transmission of FM slow scan video signals and AM single sideband audio signals over an ordinary telephone transmission line is described in the Kleinerman et al. U.S. Pat. No. 3,873,771 assigned to Telescan Communications Systems, Inc. The ITT Picture Phone System is described in the Corcoran U.S. Pat. No. 4,015,115. A further description of the slow scan method and vidicon cameras having low scanning speed is found in the Sawazaki et al. U.S. Pat. No. 3,352,966 describing a television telephone system.
A fourth group of patents relate to the early television technology patents also found in the Subclasses related to television/telephone systems. These patents are perhaps the patent references of greatest interest in the prior art with respect to the present invention. The differences of the early television technology patents from the present invention are as follows.
The Ives U.S. Pat. No. 2,099,115 issued in 1937 describes a communication system with separate two way telephone and television channels. The video signal is generated by a light source, a mechanical rotating scanning disk with apertures arranged in a spiral for scanning a beam from the light source across the field of view, and photocells that generate the serial analog scanning signal containing the light amplitude image information. The 40 kHz video signal generated by Ives requires a broad band channel for transmission. Such a channel would require a coaxial cable or, as suggested by Ives at page 6, column 2, lines 66-71, a radio link or radio channel. Another mechanical scanning disk system at the receiving station reproduces image information and is maintained in synchronism with the mechanical scanning disk at the transmitting station. It appears that transmitting and receiving scanning disks are provided at each station.
The narrow band audio signal is transmitted over a separate telephone line or telephone channel. According to Ives the telephone line is used for the speech path only. There is no suggestion of using the telephone line for transmitting the video signal also. In fact Ives was correct in assuming that the wideband analog video signal developed by his system required a separate broadband channel. The simple wire telephone line used at the time of Ives was effectively bandwidth limited by the high frequency noise of a battery/wire/ground system.
Continuing developments in telephone lines have further reduced the bandwidth. Sir Oliver Heavyside's discovery that inductance in a telegraph or telephone line could be used to reduce high frequency noise and distortion lead to his invention of the loaded line. Choke coils periodically coupled into the line interact with the resistance and capacitance of the line effectively to provide a distributed low pass filter which cuts off high frequency noise and distortion. As a result the bandwidth of contemporary conventional telephone lines is typically 0-4 kHz.
The Beltrami U.S. Pat. No. 3,263,027 issued in 1966 describes a more recent use of a Nipkow disk in a "Simultaneous Bilateral Televideophonic Communication System". Beltrami uses a cathode ray tube (CRT) as a flying spot scanner to illuminate and scan an object. A photocell generates the analog serial scanning signal encoding the light amplitude image information. The bandwidth of the Beltrami video signal is in the order of 1 MHz.
The 1 MHz bandwidth signal of Beltrami requires a broadband transmission channel which Beltrami states can be wire or wireless. Beltrami specifically suggests an antenna linked radio channel for videotelephonic transmission. It is a broadband RF AM system of transmission. For a wire link, a dedicated coaxial cable would be required because the Beltrami video signal cannot be transmitted over a conventional telephone wire.
The only use of the single Nipkow scanning disk in the Beltrami system is to generate a sync signal for both the transmitting and receiving stations. The sync signal is used to maintain the CRT scanners in synchronism at the two locations. The CRT scanner pickup and receiver generate and utilize the video signal that requires a transmission channel bandwidth in the MHz range.
Another television system using mechanical rotating scanner disks with apertures in a spiral array for the camera pickup and receiver is described in the Green U.S. Pat. No. 2,095,360 issued in 1937. The Green system clearly requires and uses a broad band radio transmission channel. Green uses a single side band radio system with a final image band of 85 kHz on a broadband carrier.
The Wright U.S. Pat. No. 2,314,471 describes another television and speech transmission system with two channels. The telephone is operated over a separate telephone line path. The video system uses a mechanical scanning disk camera pickup at each station and a CRT receiver. The video signal in the MHz range is transmitted over a broadband channel including repeater stations. The Nicolson U.S. Pat. No. 2,125,006 describes a television communicator using a broadband radio transmission system for the television signal which is broadcast over antenna wires. The sound signal is broadcast over a separate channel. The early Zworykin television described in U.S. Pat. No. 2,017,883 issued in 1935 is an all electronic TV system using CRT scanners and video signals in the MHz range.
The earlier Ives television system in U.S. Pat. No. 1,932,253 of 1933 is a two channel system. The telephone and sync signals are transmitted over a telephone wire while the picture signal is transmitted over radio channels. Alternatively it appears that a dedicated wideband trunk line may also interconnect the stations. Ives uses a single scanning disk with apertures in spiral form at each station for both the camera pickup and receiver.
Other rotating scanning disk television systems are described by Baird in U.S. Pat. Nos. 1,980,150 and 2,056,761. The latter patent also describes the use of a rotating mirror drum for the mechanical scanning apparatus illustrated in FIGS. 3, 4 and 5. For transmission of the image signals, a trunk circuit must be modified and dedicated for high bandwidth transmission. The earlier Baird U.S. Pat. No. 1,980,150 is effectively a slow scan picture transmission system which cannot operate in real time. The Finch U.S. Pat. No. 2,048,604 describes an early teletype system for printing a picture. Slow scan signals are transmitted over a telephone line to control an electromechanical printer and drum. Also of related interest are the Parker U.S. Pat. No. 1,805,594 issued in 1931 and the early Hoglund U.S. Pat. No. 1,030,240 issued in 1912 which also appear to describe low resolution, slow scan, effectively still image picture phones which do not appear capable of operating in real time.
Further background on the history of video telephone efforts can be found in the book The Video Telephone "Impact of a New Era in Telecommunications", by Edward M. Dickson and Raymond Bowers, Praeger Publishers, 1974. This book publishes the results of a study and technology assessment of video telephone research and development conducted by the Cornell University Program on Science, Technology and Society funded by RANN and NSF.